Automatic spraying techniques have long been employed for painting large articles such as cars, trucks, refrigerators, etc. The items being sprayed are generally advanced along a conveyor line which passes through a water wash paint spray booth where a fine spray of paint is directed at the articles being painted from spray guns which are located at the sides of the conveyor. Overspray paint, that is, paint which does not contact the article being painted, forms a fine mist of paint in the air space surrounding the painted article. This paint mist must be removed from the air. To accomplish this, the contaminated air is pulled through the paint spray booth by air exhaust fans. A curtain of circulating water is maintained across the path of the air in such a manner such that the air must pass through the water curtain to reach the exhaust fans. As the air passes through the water curtain, the paint mist is "scrubbed" from the air and carried to a sump basin usually located below the paint spray booth. In this area, the paint particles are separated from the water so that the water may be recycled and the paint particles disposed of.
One of the difficulties with recovering paint overspray in a water wash spray booth as described above is the limited amount of paint which can be incorporated into the water. Paint is a tacky material and it tends to flocculate and adhere to the spray booth surfaces, particularly in the sump and drain areas and must constantly be removed from the sump to prevent clogging of the sump drain and recirculating system. This is generally accomplished by shoveling which is costly and time consuming. Although it is known in the art to add detackifiers to the circulating water system to detackify the paint so that it will not adhere to the surfaces of the spray booth and be easier to remove from the system, the paint content in the circulating water system must still be kept relatively low to prevent paint flocculation. Typically, the circulating water systems with conventional detackifiers contain from about 1 to 5 percent by weight paint overspray based on total weight of the circulating water.
It is known in the art, for example, in U.S. Pat. No. 4,854,947 and U.S. Pat. No. 4,919,691, to use oil-in-water emulsions in the circulating water to scrub and to detackify the paint overspray. By using these oil-in-water emulsions, a higher percentage of paint overspray can be incorporated into the circulating water without the paint overspray deflocculating and clogging the sump drain and sewer system. The processes described in the prior art require a continuous monitoring of the pH of the bath to insure for stability of the emulsion and then chemical manipulation of the emulsion to break the emulsion and recover the various components of the paint overspray.
It is also known in the art in U.S. Pat. No. 5,019,138 to use an organic solvent, i.e., N-methyl pyrrolidone, to scrub the paint overspray from the surrounding air. However, the use of high quantities of organic solvent needed for this process is expensive and a potential health hazard.